Communication of digital information--such as data--over analog transmission links--such as telephone lines and trunks--is well known in the art. At the transmitting end of the link, a modem uses a representation of the digital information to modulate a carrier frequency and transmits the modulated carrier frequency on the transmission link. At the receiving end of the link, another modem demodulates the carrier frequency to retrieve the representation of the digital information, and reconstitutes the digital information therefrom. The pair of modems engage in a communication protocol that includes control signalling to set up and tear down the communication connection over the link between them.
Modems that handle only digital information are referred to as data-only modems. In the recent past, voice-and-data modems have made their appearance. They create a voice communication channel alongside the data communication channel such that both channels appear as a single communication to the transmission facilities that interconnect the modems. The voice channel is typically created by modulating a second carrier frequency with a representation of the voice signals. An illustrative example thereof is disclosed in U.S. Pat. No. 5,081,647. An alternative voice-and-data modem implementation is digital, wherein the voice signals are encoded into digital form and the encoded voice and data are multiplexed for transmission to the other side.
Telephone networks have traditionally provided a rich set of features for voice telephone calls. One of these features is call waiting. This feature involves interrupting of an ongoing first call with an audible indication that a second call has arrived for one of the parties to the first call. The called party is then given a chance to put the first call on hold or end it, and answer the second call.
As the use of telephone networks has expanded beyond carrying voice calls--to the carrying of data calls via modems, for example--it has become desirable to extend the voice-call features to these other types of calls as well. Accordingly, data-only modems have been modified to support the call waiting feature for data calls. Illustrative examples thereof may be found in U.S. Pat. Nos. 4,852,151 and 5,287,401. However, effectively providing call waiting to multi-media calls, such as voice-and-data calls, is a much more complex matter. This is because multiple channels of communication are involved in the call, and so their handling must be properly coordinated in providing the feature. As a consequence, the call-waiting feature has heretofore not been supported by voice-and-data modems.